This video demonstrates how to use Tear Mender Fabric glue to remove old table tennis glue residue from used table tennis rubbers. This glue makes it easy to get the old residue out from the sponge and allows you to reglue the rubbers and continue using them on the same blade or others. This method is very simple and useful. It can also be used to fix a mistake when initially gluing a rubber.
Once you have removed your old rubber from the blade, you put one or more layers of Tear Mender Fabric Glue on the sponge. Make sure to cover the whole sponge surface in a uniform and thick layer of glue. Then you let it dry.
Once the Tear Mender glue has dried completely, you can start to peel it up from one of the corners or bottom of the rubber sheet. You should be able to pull it up in large parts, but make sure to be gentle so you do not damage the sponge.
It will be easier to do this on hard-sponged rubbers than soft-sponged rubbers. You may have an easier time with two layers of glue, but you can test it out. I believe this is the easiest way to remove old glue and I highly recommend trying it out.